1. The Field of the Invention
The present invention relates to the field of telecommunications, and in particular to methods for reporting channel quality information in a wireless telecommunication network. The invention also relates to devices suitable for providing the above method.
2. The Relevant Technology
Communication over a time-varying radio channel, i.e., a real radio channel, is subject to radio channel disturbances such as additive white Gaussian noise (AWGN), flat and frequency-selective fading, and log-normal shadow fading. These disturbances introduce losses in the received information and degrade quality of the delivered service.
Different applications require different quality of service (QoS) levels: what is good for voice may not be good for video, for example. To ensure that the QoS for a specific application is met under varying radio channel conditions, radio channel adaptation techniques are useful. This involves implementing radio channel quality measurement and control. Measurement of the radio channel quality comprises estimation of one or more of radio link measures such as the received signal strength (RSS), the signal-to-noise ratio (SNR), signal-to-interference-plus-noise ratio (SINR), the bit-error-rate (BER) etc. The control part of radio channel adaptation involves adapting the modulation, coding, and/or power of the transmitted signal within system capabilities and constraints based on the radio channel quality measurements.
In some of the present telecommunication systems, mobile terminals send to base station messages containing information about the quality of the transmission channel, this information includes a Channel Quality Indicator (CQI), i.e., a value that results from the radio link measures.
In particular, 3GPP committee has worked on the standardization of the evolution of the UMTS mobile communications system denominated Long Term Evolution (LTE) that uses CQIs. These system will also use orthogonal frequency division multiplexing (OFDM) based techniques for the radio interface, i.e., a multicarrier transmission technique of transmitting information in parallel over multiple subcarriers. OFDM brings significant advantages to the radio transmission techniques of mobile wireless broadband systems, one over the other is the reduced intersymbol interference in the multipath propagation environments.
In both downlink and uplink, a basic scheduling unit is denoted as a resource block. In LTE, a resource block is defined as 7 or 6 consecutive OFDM symbols in the time domain depending on the cyclic prefix length (normal or extended) and 12 consecutive subcarriers (180 kHz) in the frequency domain.
One of the advantages of using OFDM in the LTE radio interface is the possibility of supporting frequency selective scheduling (FSS) based on the CQI provided by the mobile terminal and the estimations performed by the base station based on the sounding reference signals (SRSs) sent by the mobile terminal. This feature allows to take advantage of the multipath propagation conditions that are common in mobile communications. The base station, based on the CQI reported by the mobile terminal (for the downlink) and its own channel estimation from the reception of the SRSs (for the uplink), select the Modulation and Coding Scheme (MCS) to be used and the specific time/frequency resources of the subframe assigned to each mobile terminal. LTE also supports scheduling mechanisms that provide frequency diversity gains, as, for example, with frequency hopping, when frequency dependent scheduling is not adequate because the propagation channel exhibits a large delay spread.
The most likely operating conditions that stress LTE system capabilities is the deployments in dense urban areas with a high density of base station. In these environments the system is dimensioned (i.e., the number of base stations to be installed) more on capacity requirements rather than maximum coverage distance, as a consequence, the distance between base station is relatively small (as low as 150-200 meters). Similar conditions rise when LTE systems are deployed specially in indoor environments via the use of picocells and/or femtocells.
As a consequence, due to restrict distance between the transmitter and the receiver, wave propagation is mainly accomplished without obstacles, i.e., with a higher proportion of line of sight propagation.
It is considered that the sophisticated, frequency selective scheduling mechanisms proposed for LTE are less effective in this kind of environments due to the fact that the coherence bandwidth of the propagation channels is relatively large with respect to the system bandwidth, reducing the opportunistic gain associated with the aforementioned mechanisms.
The coherence bandwidth is a statistical measurement of the range of frequencies over which the channel can be considered “flat”, in other words the signal spectral components falling outside the coherence bandwidth will be affected differently by the transmission through the channel, compared with those components contained within the coherence bandwidth.
Most of the evaluations of LTE scheduling algorithms are based on the use of standardized channel models that have a coherence bandwidth smaller than 1.5 MHz. However, estimations carried out in realistic simulation environments, using 3D cartography and real sites locations, show that in urban areas coherence bandwidth significantly exceeds this value.
Patent application US 2009/0257356 A1 discloses a CQI including a report indicative of a value of a first channel quality metric related to one or more resource blocks, and a second report indicative of the value of the channel quality metric over the entire channel. The channel quality metric may be a signal to interference plus noise ratio (SINR), and the second channel quality report may be indicative of a difference between a mean value of the SINR of the resource blocks exceeding a predetermined threshold and a mean value of the SINR over the entire channel.
The CQI reporting method of this patent application makes use of a bit mask where each bit takes a value of 0 or 1 if the CQI of the resource block is higher or lower than an established threshold. Therefore the size of this bit mask in bits is equal to the number of resource blocks.
This method provides the network with good information on channel quality over a big frequency spectrum, yet it requires a lot of radio resources for CQI reporting.